Aluminum-magnesium alloy



Patented Oct. 19, 1937 -.UNITED STATES PATENT OFFICE.

ALUMINUM-MAGNESIUM ALLOY No Drawing. Application February 14, 1936,

Serial No. 63,942

9 Claims. (Cl. 75-147) This invention relates to castings of aluminum base alloys containing from about 1 to 11 per cent magnesium, and it is particularly concerned with rendering such castings leak proof to liquids and gases under pressure.

Castings made from aluminum-magnesium alloys have been found to possess cert'ain highly desirable characteristics such as low specific gravity, relatively high tensile strength and excellent corrosion resistance. For this reason these alloys have been used where the service requirements emphasized these properties and where other aluminum base alloys have not proved. to be entirely satisfactory. The high strength of these alloys coupled with good corrosion resistance and machinability has made them especially useful in the manufacture of certain parts in which liquids or gas are retained under pressure. While the castings are strong enough to easily withstand the stresses imposed upon them by the pressure of the fluid medium, yet they have been found to leak occasionally,

that is, the fluid passes through the casting. A

penetration of the metal by the fluid is not considered to be leakage, but there must be a visible and tangible escape through the metal wall to constitute leakage in the sense in which that term is herein employed. This leakage is very undesirable and can not be permitted at all in some cases; The object of my invention is to greatly reduce such leakage if not entirely prevent it without materially diminishing the strength of the alloy or impairing its casting qualities.

My invention is predicated on the discovery that the addition of from 0.3- to 1.0 per cent of tin to aluminum-magnesium alloys containing from 1 to 11 per cent magnesium efiectively diminishes the leakage referred to above. Since leakage is a function of the applied pressure when other conditions remain unchanged, it is obvious that a casting may withstand a pressure of say twenty-five (25) pounds per square inch without leaking and yet permit the passage of some gas or liquid at a pressure of one hundred ('100) pounds per square inch. The addition of tinin anycase does effect a marked reduction in leakage and renders an alloy, which might otherwise fail in this respect, useful for resistance to pressure. The amount of tin required to accomplish this purpose varies with the alloy and the pressure to be encountered. In general a larger amount of tin is desirable in alloys having a magnesium content 01-9 or 10 per cent than in those containing only} or 4 per cent of the latter element and also where relatively high pressures are to be encountered. At least 0.3 per cent of tin is needed to improve the leakproofness of the alloys and more than 1 per cent impairs the strength and corrosion resistance of 5 the alloy to an undesirable extent. For most purposes a range of 0.4 to.0.6 per cent is preferred. The magnesium content of the alloys is limited by commercial utility, less than 1 per cent and more than 11 per cent not having desirable physical properties. 7

The tin is effective in both the as-cast and heat treated alloys. In general alloys containing up to-5 to 6 per cent magnesium are not heat treated whereas those containing 9 or 10 per cent are usually thermally treated to develop their full strength. r

The addition of 0.3 to 1 per cent of tin to the aforementioned aluminum-magnesium alloys does not excessively reduce their physical properties. For example, asand cast alloy composed of aluminum and 5.25 per cent magnesium in the as-cast condition has a tensile strength of 27,000 pounds per square inch, a yield strength of 15,000 pounds per square inch and an elongation of 8 per cent, whereas the same alloy plus 1 per cent tin has a tensile strength of 24,000 pounds per square inch, a yieldstrength of 14,000 pounds per square inch and an elongation of 5 per cent. Similarly a cast alloy of aluminum and 10 per cent magnesium which had been heat treated 16 hours at 810 F. and quenched has a tensile strength of 44,000 pounds per square inch, a yield strength of 25,000 pounds per square inch and an elongation of 13 per cent. The same 211- 5 loy plus 0.4 per cent tin and under the same conditions of heat treatment has a tensile strength of 42,000 pounds per square inch, a yield strength of 24,000 pounds per square inch and an elongation of 12 per cent.

One type of leak test which has been used to determine the relative merits of various alloys consists of casting a cup shaped article, mounting it on a fixture, applying a soap solution to the exterior and applying air pressure to the 45 interior of the cup. Any leakage will be manifested by soap bubbles appearing on the external surface of the cup. A cup made from an alloy of aluminum and 5.25 per cent magnesium leaked badly under an air pressure of 50 pounds per 5 square inch, while there was scarcely any leakage through a cup made from the same alloy plus 1 per cent tin when tested under the same conditions.

Breierred embodiments of my invention are to 55 be found in aluminum base alloys containing 3.75 per cent magnesium and 0.4 per cent tin; 5.25 per cent magnesium and 0.5 per cent tin; 10 per cent magnesium and 0.5 per cent tin, also 10 per cent magnesium and 1 per cent tin.

Small amounts of certain elements other than magnesium and tin may be present in the alloy as impurities or as intentional additions. Iron and silicon, for example, are always present as impurities in varying I amounts. These are preferably kept below a maximum total of 0.6 per cent. On the other hand, there are elements which must be avoided if satisfactory castings are to be made. Theelement phosphorus, in particular, should be avoided since it embrittles the alloy; Other elements, such as chromium, manganese and titanium, are useful and may be added in small amounts to enhance particular properties of the alloy without destroying its fundamentalcharacteristics and resistance to leakage. poses of my invention these elements constitute a group of substances which have a similar efiect upon the aluminum-magnesium-tin alloys herein described. When chromium or manganese is used, each one may be added in amounts of about 0.1 to 1 percent, although it is preferable to employ 0.5 per cent or less. Where titanium is added, only 0.01 to 0.15 per cent is required. If two or more of these elements are to be employed the total amount should in no case exceed about 1 per cent.

A particular application for alloys of the type described her'einabove may-be found in their use as a material for rubber tire'molds. The tires are cured in these molds through the application of heat, and hence aluminum base alloys are particularly suitable for this purpose because of their high thermal conductivity coupled with lightness in weight; The molds are heated by means of steam passed through chambers within the mold walls, the steam being under a pressure of about pounds per square inch. It is obviously desirable to confine the steam to the heating chambers and hence the need for a substantially leak proof alloy.-

The leak proof alloy also finds use in heat exchanger plates in pasteurizing units of milk treating equipment. Here again steam or cooling water under pressure must be confined to chambers within the plates.

The tin may be added to the alloy in accordance with the usual foundry practice. The prac- For the pur-.

tices followed in casting aluminum-magnesium alloys are applicable "to' my improved alloy. The

aluminum to be employed in making the alloy should be of a good commercial grade, the higher purity metal being preferred.

The term aluminum as used herein and in the appended claims embraces the usual impurities found in aluminum ingot of commercial grade or picked up in the course of the ordinary handling operations incident to melting practice.

Having thus described my invention and the manner in which the same is to be performed, I claim:

1. An aluminum base alloy consisting of 1 to 11 per cent magnesium, 0.3 to 1 per cent tin, and the balance aluminum.

2. An aluminum base alloy consisting of from 1 to 11 per cent magnesium, 0.3 to 1.0 per cent tin, a total not exceeding 0.6 per cent of iron and silicon as impurities, and the balance aluminum.

8. An aluminum base alloy consisting of 1 to 11 per cent magnesium, 0.3 to 1 per cent tin and a total of 0.05 to 1 per cent of hardening metal from the group composed of chromium, manganese and titanium, the titanium, if present, being not in excess of 0.15 per cent, the balance of the alloy being aluminum.

4. An aluminum base alloy consisting of 1 to 11 per cent magnesium, 0.3 to 1 per cent tin and.

hardening metal selected from the group composed of 0.1 to 1 per cent chromium, 0.1 to 1 per cent manganese and 0.05 to 0.15 per cent titanium, the total amount of said hardening metal not exceeding about 1 per cent, the balance of the alloy being aluminum.

5. An aluminum base alloy consisting of from 1 to 6 per cent magnesium, 0.3 to 1 per cent tin and the balance aluminum.

6. An aluminum base alloy consisting of from IO per cent magnesium, 0.5 per cent tin and the balance aluminum.

WALTER E. SICHA. 

